The present invention relates to an improved technique for recovering shale oil from oil shale.
Shale oil is composed of inorganic matter (rock) and organic matter called kerogen. As is well known, when oil shale is heated (retorted) at elevated temperatures on the order of 600.degree. F. to 900.degree. F. in the absence of significant oxygen, kerogen is destructively distilled (pyrolyzed) to form a hydrocarbon gas, shale oil and carbon. The shale oil being at elevated temperature is in the vapor phase while the carbon is in the form of coke. Continued heating of shale oil will cause decomposition to form more gas and more coke.
A compilation of recent studies have shown that the yield of shale oil possible from retorting oil shale is dependent upon a number of variables. In particular, it has been found that the yield of shale oil will be maximized if the following four criteria are met:
(1) retorting is accomplished at low pressures, preferably on the order of 1 atmosphere pressure;
(2) the oil shale is heated-up from ambient to maximum temperature during retorting as quickly as possible;
(3) the maximum temperature during retorting is on the order of 800.degree. F. to 900.degree. F. (425.degree. C. to 485.degree. C.); and
(4) the shale oil obtained from the decomposition of kerogen is removed from the oil shale and cooled as quickly as possible. For more thorough information on the pyrolysis of oil shale, see Wise, et al., A LABORATORY STUDY OF GREEN RIVER OIL SHALE RETORTING UNDER PRESSURE IN A NITROGEN ATMOSPHERE, The Laramie Energy Research Center, Energy Research and Development Administration, Laramie, Wyo. LERC/TPR-76/1; Bae, SOME EFFECTS OF PRESSURE ON OIL-SHALE RETORTING, Society of Petroleum Engineers Journal, Sept. 1969; Campbell, et al., DYNAMICS OF OIL GENERATION AND DEGRADATION DURING RETORTING OF OIL SHALE BLOCKS AND POWDERS, from the Proceedings of the Tenth Oil Shale Symposium, Colorado School of Mines, Apr. 1977; and Needham, OIL YIELD AND QUALITY FROM SIMULATED IN-SITU RETORTING OF GREEN RIVER OIL SHALE, 51st Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, Oct. 1976 SPE 6069.
Beginning in the 1920's, numerous techniques have been proposed for processing oil shale in situ to recover shale oil therefrom. The first such proposal, referred to as "true in situ combustion retorting", involved the in situ retorting of the oil shale. Heat necessary for retorting was to be supplied by in situ combustion, combustion being accomplished along a combustion front which moved from one end of the bed to the other during the retorting operation.
The true in situ combustion retorting technique was first tried in the 1950's and was attempted a number of times in the 1950's and the 1960's. In carrying out this process, small fissures were introduced into the oil shale bed by hydrofrac techniques prior to retorting in order to expedite the passage of vaporous shale oil out of the bed being processed. Unfortunately, the true in situ combustion retorting technique was not successful.
In the early 1970's, a modification of the true in situ combustion retorting technique was first tried. This technique, referred to as the "modified in situ combustion retorting technique" differs from the true in situ combustion retorting technique in that prior to retorting, partial mining around the bed is accomplished to provide a greater flow path for the escape of the shale oil. Also prior to retorting, the shale oil bed is broken up or fragmentized (referred as "rubblized") into chunks or pieces, this usually being accomplished by means of explosives.
In practice it was found that the modified in situ combustion retorting technique was able to recover shale oil in amounts as high as 60% of theoretical yield when practiced on beds on the order of 65,000 to 140,000 cubic feet. However, when tried on beds on the order of 4 million cubic feet, yields dropped off to around 30% of theoretical. Although the exact reason for this is not known, it is theorized that this low conversion was due to the fact that the fire went out in various spots in the combustion zone as it moved through the bed, which in turn was due to the significant variations in the kerogen content in oil shale. Increasing non-uniformity of fragmentation of the oil shale with increasing bed size is also believed to contribute to the low yields obtained.
In addition to combustion retorting, other techniques have been proposed for the recovery of shale oil from oil shale by the in situ retorting of oil shale. Many of these techniques are based on utilization of electrical energy for heating of the oil shale. Heat generation through induction heating of electrodes, induction heating of the oil shale itself and heating through the application of VHF and UHF energy have all been proposed. These various techniques as well as the disadvantages associated therewith are summarized in U.S. Pat. No. 4,144,935.
Still another method for the in situ recovery of shale oil from oil shale was proposed in the mid-1970's. This technique is an offshoot of the true in situ combustion retorting technique and uses radio frequency energy rather than combustion to furnish the heat necessary for retorting. In accordance with this technique, a grid of electrodes is arranged to bound (in an electrical sense) the bed to be retorted on at least two sides and radio frequency energy applied to the grid to cause dielectric heating of the kerogen in much the same way as a microwave oven heats its contents.
This technique (known as the IITRI technique for the assignee thereof, Illinois Institute of Technology Research Institute) appears to have many advantages over the above-mentioned techniques. Regarding previously proposed techniques based on the use of electrical energy, the IITRI technique appears to be much more efficient. Regarding in situ combustion techniques, the IITRI technique avoids the use of a combustion front and hence the various disadvantages associated with a combustion front particularly the possibility of oxygen coming into contact with shale oil vapors, are also avoided.
The IITRI technique, however, has not as yet been reduced to practice. It has, however, been shown in the laboratory on an extremely small sample of oil shale that shale oil can be recovered using radio frequency energy.
Although the IITRI technique appears to be theoretically possible, it is believed that the maximum possible yields of shale oil possible when using this technique will not be as great as expected. Accordingly, it is an object of the present invention to provide a modification of the IITRI technique which will allow shale oil to be recovered in greater amounts than possible in accordance with the presently proposed IITRI process and will allow the IITRI process to be practiced at reasonable cost effectiveness.